1. Field of the Invention
The present invention relates to a method of detecting a termination time of a surface layer removal process for removing a surface layer formed on a substrate thus is useful for etching of a semiconductor substrate or a substrate for a liquid crystal device (hereinafter referred to as "wafer").
2. Description of the Prior Art
In an etching step for manufacturing a semiconductor device, for example, it is necessary to correctly detect a termination time at which etching of a desired region of a semiconductor wafer is terminated thereby to remove the wafer from an etching solution etc. This is because side etching to peripheral regions etc. is inevitably increased if the wafer is dipped in the etching solution or the etching solution is sprayed onto the wafer surface over a required time. Well-known examples of conventional termination detecting methods are as follows:
In a first method, an infrared projector and a light receiver are provided on both sides of a wafer. Output Q.sub.1 (FIG. 1) of the light receiver is processed by a logarithmic amplifier, a peak value detecting circuit, a filter circuit and a differentiating circuit (all not shown) to become a signal Q.sub.2. Then, any time change of the signal Q.sub.2 is observed.
It is assumed that the signal Q.sub.2 rises at a time t.sub.1 and then falls at a time t.sub.2 upon removal of an aluminum layer on a surface through progress of etching. In this case, a time t.sub.3 after lapse of a time period proportional to (t.sub.2 -t.sub.1) from the point t.sub.2 is detected as an etching termination time.
In a second method, a value obtained by smoothing (averaging) a difference value of output Q.sub.1 from a light receiver by a prescribed time constant is employed as an index for processing termination detection in optical system arrangement similar to the above. Approach of the time to terminate etching is recognized by detecting a drop in the index.
In a third method, an initial value of a light receiving output at an etching starting time is stored. A particular moment of time after a lapse of a prescribed time period from an origination of a level change of the light receiver output from the initial value is assumed to be a processing termination time.
Although these methods have their own respecive characteristics, all of the same are applicable only to the case of etching a surface layer, such as an aluminum layer, which transmits no light. This is because no rise of light receiver output upon termination of etching occurs when etching a light-transmissible surface layer.
A fourth method, which is disclosed in PCT/US83/00655, relates to a termination detection technique for an etching material having light transmissibility in a specific wavelength band. In this method, a region substantially larger than that of an etched portion is subjected to light irradiaion. Then, obtained is an optical interference signal of light surface-reflected by the etching layer as etching progresses, with light transmitted through the etching layer and reflected by the surface layer of a supporting substrate. Secondary and tertiary time differentiations are performed on the signal. Etching is stopped upon lapse of a time period from a reference time at which both of the differentiation signals are substantially zeroed, this time period is substantially equal to or is twice as much as a time up to the reference time.
In this method, however, a problem is caused when surface treatment is performed by spraying a surface treating solution from a nozzle to the surface of a wafer while rotating the wafer. In the case of such a processing system, a slight time change may be recognized in sensor output even if etching is completed. The sensor output is not converged to zero in such a case. Thus, the fourth method cannot be applied to that case since the same is based on the premise that the sensor output is converged to zero.